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  <h2>python/前端/10-JavaScript函数</h2>



  <p class="post-date">2020-11-03</p>
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    <section class="markdown-content"><h3 id="1-1-声明函数的方式"><a href="#1-1-声明函数的方式" class="headerlink" title="1-1 声明函数的方式"></a>1-1 声明函数的方式</h3><ul>
<li>function 关键字</li>
<li>匿名函数方式(表达式方式)</li>
<li>Function 构造函数方式</li>
</ul>
<h3 id="1-2-参数问题"><a href="#1-2-参数问题" class="headerlink" title="1-2 参数问题"></a>1-2 参数问题</h3><ul>
<li>形参和实参数量问题</li>
<li>可选形参(参数默认值)</li>
<li>可变长的实参列表:实参对象 aruguments</li>
</ul>
<h3 id="1-3-回调函数"><a href="#1-3-回调函数" class="headerlink" title="1-3 回调函数"></a>1-3 回调函数</h3><p>一个函数就可以接收另一个函数作为参数，这种函数就称之为回调函数(高阶函数)</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">add</span>(<span class="params">x, y, f</span>) </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> f(x) + f(y);</span><br><span class="line">&#125;</span><br><span class="line">add(<span class="number">-5</span>, <span class="number">6</span>, <span class="built_in">Math</span>.abs)</span><br></pre></td></tr></table></figure>

<h3 id="1-4-递归函数"><a href="#1-4-递归函数" class="headerlink" title="1-4 递归函数"></a>1-4 递归函数</h3><p>函数内部调用自己就是递归函数，</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//用递归 实现阶乘</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">multiply</span>(<span class="params">n</span>) </span>&#123;</span><br><span class="line">    <span class="keyword">if</span> (n == <span class="number">1</span>) &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="number">1</span></span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> n * multiply(n - <span class="number">1</span>)</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="1-5-自调函数"><a href="#1-5-自调函数" class="headerlink" title="1-5 自调函数"></a>1-5 自调函数</h3><p>函数生声明完 直接调用</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'ok'</span>)</span><br><span class="line">&#125;)()</span><br></pre></td></tr></table></figure>

<h3 id="1-6-闭包函数"><a href="#1-6-闭包函数" class="headerlink" title="1-6 闭包函数"></a>1-6 闭包函数</h3><p>当一个函数返回了一个函数后，其内部的局部变量还被新函数引用，形成闭包</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">count</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="keyword">var</span> arr = [];</span><br><span class="line">    <span class="keyword">for</span> (<span class="keyword">var</span> i=<span class="number">1</span>; i&lt;=<span class="number">3</span>; i++) &#123;</span><br><span class="line">        arr.push((<span class="function"><span class="keyword">function</span> (<span class="params">n</span>) </span>&#123;</span><br><span class="line">            <span class="keyword">return</span> <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">                <span class="keyword">return</span> n * n;</span><br><span class="line">            &#125;</span><br><span class="line">        &#125;)(i));</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> arr;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> results = count();</span><br><span class="line"><span class="keyword">var</span> f1 = results[<span class="number">0</span>];</span><br><span class="line"><span class="keyword">var</span> f2 = results[<span class="number">1</span>];</span><br><span class="line"><span class="keyword">var</span> f3 = results[<span class="number">2</span>];</span><br><span class="line"></span><br><span class="line">f1(); <span class="comment">// 1</span></span><br><span class="line">f2(); <span class="comment">// 4</span></span><br><span class="line">f3(); <span class="comment">// 9</span></span><br></pre></td></tr></table></figure>

<h2 id="2、JavaScript-作用域"><a href="#2、JavaScript-作用域" class="headerlink" title="2、JavaScript 作用域"></a>2、JavaScript 作用域</h2><h3 id="2-1-局部作用域"><a href="#2-1-局部作用域" class="headerlink" title="2-1 局部作用域"></a>2-1 局部作用域</h3><p>函数中使用定义的变量就是局部变量，只能在本函数中使用</p>
<h3 id="2-2-全局作用域"><a href="#2-2-全局作用域" class="headerlink" title="2-2 全局作用域"></a>2-2 全局作用域</h3><p>在，函数外面，定义的变量是全局变量。哪都可以用</p>
<p><strong>变量提升</strong></p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> a = <span class="number">100</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">demo</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(a)</span><br><span class="line">    <span class="keyword">var</span> a = <span class="number">200</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="2-3-作用域链"><a href="#2-3-作用域链" class="headerlink" title="2-3 作用域链"></a>2-3 作用域链</h3><p>函数嵌套函数会形成作用域链</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">demo</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">fn</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">        <span class="function"><span class="keyword">function</span> <span class="title">fn1</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line"></span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="2-4-块状作用域-ES6"><a href="#2-4-块状作用域-ES6" class="headerlink" title="2-4 块状作用域(ES6)"></a>2-4 块状作用域(ES6)</h3><p>使用<code>let</code>关键字声明的变量会具有块状作用域</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i &lt; <span class="number">10</span>; i ++) &#123;</span><br><span class="line"></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.log(i) <span class="comment">//变量不存在 Uncaught ReferenceError: i is not defined</span></span><br></pre></td></tr></table></figure>

<h2 id="3-函数使用案例"><a href="#3-函数使用案例" class="headerlink" title="3 函数使用案例"></a>3 函数使用案例</h2><h3 id="3-1-基本使用"><a href="#3-1-基本使用" class="headerlink" title="3-1 基本使用"></a>3-1 基本使用</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br></pre></td><td class="code"><pre><span class="line"># 在python定义函数需要用到关键字def</span><br><span class="line"># 在js中定义函数需要用到关键字function</span><br><span class="line"></span><br><span class="line"># 格式</span><br><span class="line"><span class="function"><span class="keyword">function</span> 函数名(<span class="params">形参<span class="number">1</span>,形参<span class="number">2</span>,形参<span class="number">3.</span>..</span>)</span>&#123;函数体代码&#125;</span><br><span class="line"></span><br><span class="line"># 无参函数</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">func1</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">'hello world'</span>)</span><br><span class="line">&#125;</span><br><span class="line">func1()  # 调用 加括调用 跟python是一样的</span><br><span class="line"></span><br><span class="line"># 有参函数</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">func2</span>(<span class="params">a,b</span>)</span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(a,b)</span><br><span class="line">&#125;</span><br><span class="line">func2(<span class="number">1</span>,<span class="number">2</span>)</span><br><span class="line"></span><br><span class="line">func2(1,2,3,4,5,6,7,8,9)  # 多了没关系 只要对应的数据</span><br><span class="line">VM3610:<span class="number">2</span> <span class="number">1</span> <span class="number">2</span></span><br><span class="line"><span class="literal">undefined</span></span><br><span class="line"></span><br><span class="line">func2(1)  # 少了也没关系</span><br><span class="line">VM3610:<span class="number">2</span> <span class="number">1</span> <span class="literal">undefined</span></span><br><span class="line">  </span><br><span class="line"># 关键字arguments</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">func2</span>(<span class="params">a,b</span>)</span>&#123;</span><br><span class="line">  console.log(arguments)  # 能够获取到函数接受到的所有的参数</span><br><span class="line">  <span class="built_in">console</span>.log(a,b)</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">func2</span>(<span class="params">a,b</span>)</span>&#123;</span><br><span class="line">  <span class="keyword">if</span>(<span class="built_in">arguments</span>.length&lt;<span class="number">2</span>)&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'传少了'</span>)</span><br><span class="line">  &#125;<span class="keyword">else</span> <span class="keyword">if</span> (<span class="built_in">arguments</span>.length&gt;<span class="number">2</span>)&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'传多了'</span>)</span><br><span class="line">  &#125;<span class="keyword">else</span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(<span class="string">'正常执行'</span>)</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"># 函数的返回值  使用的也是关键字return</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">index</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">  <span class="keyword">return</span> <span class="number">666</span></span><br><span class="line">&#125;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">index</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">  <span class="keyword">return</span> <span class="number">666</span>,<span class="number">777</span>,<span class="number">888</span>,<span class="number">999</span></span><br><span class="line">&#125;</span><br><span class="line">res = index();</span><br><span class="line"><span class="number">999</span></span><br><span class="line">res</span><br><span class="line">999  # 只能拿到最后一个</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">index</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">  <span class="keyword">return</span> [<span class="number">666</span>,<span class="number">777</span>,<span class="number">888</span>,<span class="number">999</span>]</span><br><span class="line">&#125;</span><br><span class="line"># js不支持解压赋值</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"># 匿名函数  就是没有名字</span><br><span class="line"><span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">'哈哈哈'</span>)</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">var</span> res = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">'哈哈哈'</span>)</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"># 箭头函数(要了解一下)  主要用来处理简单的业务逻辑 类似于python中的匿名函数</span><br><span class="line"><span class="keyword">var</span> func1 = <span class="function"><span class="params">v</span> =&gt;</span> v;  <span class="string">""</span><span class="string">"箭头左边的是形参 右边的是返回值"</span><span class="string">""</span></span><br><span class="line">等价于</span><br><span class="line"><span class="keyword">var</span> func1 = <span class="function"><span class="keyword">function</span>(<span class="params">v</span>)</span>&#123;</span><br><span class="line">  <span class="keyword">return</span> v</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> func2 = <span class="function">(<span class="params">arg1,arg2</span>) =&gt;</span> arg1+arg2</span><br><span class="line">等价于</span><br><span class="line"><span class="keyword">var</span> func1 = <span class="function"><span class="keyword">function</span>(<span class="params">arg1,arg2</span>)</span>&#123;</span><br><span class="line">  <span class="keyword">return</span> arg1+arg2</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="3-2-作用域"><a href="#3-2-作用域" class="headerlink" title="3-2 作用域"></a>3-2 作用域</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br></pre></td><td class="code"><pre><span class="line"># 跟python查找变量的顺序一致</span><br><span class="line"><span class="keyword">var</span> city = <span class="string">"BeiJing"</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="keyword">var</span> city = <span class="string">"ShangHai"</span>;</span><br><span class="line">  <span class="function"><span class="keyword">function</span> <span class="title">inner</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">    <span class="keyword">var</span> city = <span class="string">"ShenZhen"</span>;</span><br><span class="line">    <span class="built_in">console</span>.log(city);</span><br><span class="line">  &#125;</span><br><span class="line">  inner();</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">f();  <span class="comment">//输出结果是？</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> city = <span class="string">"BeiJing"</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Bar</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="built_in">console</span>.log(city);</span><br><span class="line">&#125;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">  <span class="keyword">var</span> city = <span class="string">"ShangHai"</span>;</span><br><span class="line">  <span class="keyword">return</span> Bar;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">var</span> ret = f();</span><br><span class="line">ret();  <span class="comment">// 打印结果是？</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> city = <span class="string">"BeiJing"</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">    <span class="keyword">var</span> city = <span class="string">"ShangHai"</span>;</span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">inner</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(city);</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> inner;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">var</span> ret = f();</span><br><span class="line">ret();</span><br></pre></td></tr></table></figure>

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      <ol class="toc-nav"><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#1-1-声明函数的方式"><span class="toc-nav-text">1-1 声明函数的方式</span></a></li><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#1-2-参数问题"><span class="toc-nav-text">1-2 参数问题</span></a></li><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#1-3-回调函数"><span class="toc-nav-text">1-3 回调函数</span></a></li><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#1-4-递归函数"><span class="toc-nav-text">1-4 递归函数</span></a></li><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#1-5-自调函数"><span class="toc-nav-text">1-5 自调函数</span></a></li><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#1-6-闭包函数"><span class="toc-nav-text">1-6 闭包函数</span></a></li></ol></li><li class="toc-nav-item toc-nav-level-2"><a class="toc-nav-link" href="#2、JavaScript-作用域"><span class="toc-nav-text">2、JavaScript 作用域</span></a><ol class="toc-nav-child"><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#2-1-局部作用域"><span class="toc-nav-text">2-1 局部作用域</span></a></li><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#2-2-全局作用域"><span class="toc-nav-text">2-2 全局作用域</span></a></li><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#2-3-作用域链"><span class="toc-nav-text">2-3 作用域链</span></a></li><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#2-4-块状作用域-ES6"><span class="toc-nav-text">2-4 块状作用域(ES6)</span></a></li></ol></li><li class="toc-nav-item toc-nav-level-2"><a class="toc-nav-link" href="#3-函数使用案例"><span class="toc-nav-text">3 函数使用案例</span></a><ol class="toc-nav-child"><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#3-1-基本使用"><span class="toc-nav-text">3-1 基本使用</span></a></li><li class="toc-nav-item toc-nav-level-3"><a class="toc-nav-link" href="#3-2-作用域"><span class="toc-nav-text">3-2 作用域</span></a></li></ol>
    
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